Fix runtime error

Googler · nshahan · commit e3636ae8041e · 2019-04-15T15:42:30.000-07:00
oSuchMethodError: invalid member on null: 'selectionChanges'

PiperOrigin-RevId: 243122097
diff --git a/angular_components/lib/material_menu/menu_item_groups.dart b/angular_components/lib/material_menu/menu_item_groups.dart
@@ -535,7 +535,7 @@ class MenuItemGroupsComponent
   void _listenForSelectionChanges(MenuItemGroup group) {
     if (group is MenuItemGroupWithSelection) {
       _disposer.addStreamSubscription(
-          group.selectionModel.selectionChanges.listen((_) {
+          group.selectionModel?.selectionChanges?.listen((_) {
         _updateItemsAriaCheckedState(_menu);
       }));
     }

Original file line number	Diff line number	Diff line change
`@@ -535,7 +535,7 @@ class MenuItemGroupsComponent`
`535`	`535`	`void _listenForSelectionChanges(MenuItemGroup group) {`
`536`	`536`	`if (group is MenuItemGroupWithSelection) {`
`537`	`537`	`_disposer.addStreamSubscription(`
`538`		`- group.selectionModel.selectionChanges.listen((_) {`
	`538`	`+ group.selectionModel?.selectionChanges?.listen((_) {`
`539`	`539`	`_updateItemsAriaCheckedState(_menu);`
`540`	`540`	`}));`
`541`	`541`	`}`